1. Field of the Invention
The present invention relates generally to an optical module. More particularly, the present invention relates to a TO-can (Transistor-Outline-can) type optical module.
2. Description of the Related Art
An optical module is an essential part of any system used for optical transmission. Owing to the recent rapid growth of the information industry, there are increasing proportions of information transmission traveling over an optical communication network, as well as increased demand for fast transmission and transmission of a large volume of information. The optical module must be designed to support fast and large-volumes of information transmission. Optical devices such as a laser diode (LD) or a photodiode (PD) for the optical module are usually available in TO-can packages.
FIG. 1 is a perspective view of a conventional TO-can optical module package 100. Referring to FIG. 1, the conventional TO-can optical module package 100 comprises a stem 101 provided at a surface thereof, with a protruding heat sink block 111, and a plurality of leads 102. The four leads 102 comprise two leads for driving laser diode 103 and two leads for biasing monitor photodiode 104. The laser diode LD 103 and the monitoring photodiode MPD 104 are arranged on the surface the stem 101. In particular, LD 103 is normally arranged on the heat sink block 111. The LD 103 and the MPD 104 are connected to the leads 102 by, for example, wire bonding.
The leads 102 are coaxially aligned via through-holes 113 penetrating both surfaces of the stem 101, the through-holes 113 are filled with a glass sealant 105, and the glass sealant 105 is melted, thereby fixing the leads 102 to the stem 101 and sealing the through holes 113 at the same time. Such a conventional TO-can package is the model C-13-DFB10-TJ-SLC21 manufactured and sold by Luminent Inc.
However, the conventional optical module package is not practical for high-speed transmissions at 10 Gbps or over because of (1) parasitic inductance which is inherent in the leads, (2) parasitic capacitance between the leads and the stem, and (3) characteristic impedance mismatch for the RF signal passing through the leads.
FIG. 2 is a perspective view of another conventional TO-can optical module package 200 featuring a ceramic feed-through. Referring to FIG. 2, the TO-can optical module package 200 comprises a stem 201 provided with a protruding heat sink block 211, and a ceramic stack feed-through 203 inserted into the stem 201. The feed-through 203, disposed on the heat sink block 211, has a coplanar waveguide (CPW) 202 at a surface thereof. The CPW-type package 200 receives an external RF signal through a plurality of leads 204. TO TX PKG A2527 of Kyocera Corp. is one of such CPW-type packages.
The feed-through 203 is normally fabricated in a ceramic stack structure. Since the feed-through 203 is formed by LTCC (Low Temperature Co-fired Ceramic), its processing temperature is high, for example, between 800 and 1000° C. Thus, the manufacturing costs are higher than those of conventional TO-can optical module package shown in FIG. 1.
Moreover, when a waveguide structure is arranged with the optical module to improve the RF characteristics, the size of a sub-mount has to be increased. In this case, light emitted from the back facet of laser diode is reflected from or scattered on the surface of the sub-mount, resulting in a decrease of monitoring photocurrent. To solve this problem, Sumitomo Inc. proposed a TO-can type optical module package in which the sub-mount is shaped like “”. However, the conventional technology has distinctive shortcomings such as an increase in sub-mount manufacturing costs and a difficult assembly procedure. What makes it worse is that if a matching resistor is mounted on the sub-mount without any consideration of the other components, heat problems can become severe in case of un-cooled operation. When a mixture of a DC bias and an RF signal, produced from an external bias-tee, passes through the matching resistor, the heat dissipation mostly coming from DC current directly increases the operating temperature of the LD, which is located very close to the matching resistor, thereby deteriorates the performance of the TO-can type optical module in a fatal fashion.